The goal of this research is to evaluate the persistence of individual differences in early lexical development and to identify their predictive role in the subsequent acquisition of inflectional morphology. In Study 1, we propose to investigate three groups of children who are at the extremes in vocabulary development (determined by the MacArthur: CDI) on the following dimensions: (a) onset/rate of vocabulary development (N=24, 18-20 mos.) (slow/late vs. early/fast); (b) composition of total vocabulary (N=24, 16-18 mos) high nominals/low predicates vs. mix of nominals and predicates) and (c) composition of verb and nominal vocabularies (n=24, 18-20 mos.) (high irregulars vs. regulars). These characteristics amy foreshadow subsequent language deficits and are associated with processing or cognitive "styles" (e.g., holistic vs. analytic) that cut across language milestones. Using laboratory experimental and free-speech measures, children's use and generalization of English plural (nominal) and past tense (verbal) inflectional morphemes with familiar and nonsense words is assessed. Laboratory sessions will occur at identification (Time 1) and at one-year follow-up (Time 2). In Study 2, we propose an experimentally-guided series of connectionist-functionalist computer models in which analogous individual variations in lexical growth and composition are manipulated across training conditions. These models assume a highly interactive, non- modular organization of language and predict continuity in the mechanisms guiding the learning of individual lexical items and the generalization of morphological patterns. This approach has contributed considerable insight into the nature of the language faculty in several domains, notably a single-mechanism constraint-satisfaction account of many classic phenomena, including U-shaped development and selective deficits in productive inflectional morphology in specific language impairment (SLI). Modeling individual differences is a natural and necessary next step toward identifying the array of possible means for abstracting and generalizing regularities in linguistic input available to children who might not be approaching the task of acquisition as quickly or with the same strategic advantages as their age-mates. In Study 3, we test these hypotheses using currently available free speech and parental report data in children with Down Syndrome (DS) and SLI. In general, this research adds to our understanding of the interactive and domain-general nature of the mechanisms guiding acquisition in normal children and those who are "at risk" for developmental and language delay.